This invention relates generally to an apparatus and method for obtaining improved tractive performance of agricultural wheeled tractors without sacrificing riding comfort during field operations. Reference is made to the following U.S. Pat. Nos. 4,694,872, 4,603,916, 4,508,150, and 4,402,357 the disclosures of which are incorporated herein by reference.
In the above disclosures the performance of a pneumatic tire for a tractor's drive wheels are reviewed and methods for improving productivity and reducing soil compaction are shown.
The shortcomings of farm drive-tires during field operations are well known in the art. To offset their often suboptimum performance in soils a variety of methods and means for enhancing traction have become standard practice; such as adding weight by means of liquid ballast and cast iron wheel weights, the use of dual or triple drive tires, employing four wheel drive by means of both so-called articulated design and so-called Front Wheel Drive Assist (FWDA).
The last approach has become increasingly popular in recent decades as the trend to heavier and more powerful tractors evolved, resulting in inadequate tractive performance of the regular Two-Wheel-Drive (2WD) type farm tractor, generally due to excessive rate of tire slip, regardless of the amount of ballast weight added. However, none of these techniques has provided a complete solution. The 1986 ASAE Paper No 86-1067 (ASAE: American Society of Agricultural Engineers) provides an illuminating example of the present state of the art and is entitled COMPARATIVE EVALUATION OF FWDA to TWO-WHEEL DRIVE TRACTORS. (Prof. L. R. Shell et al., Southwest Texas State University). The paper discusses (1) the cost of the FWDA device; and (2) the tractive performance of the 2WD configuration.
In regard to (1), the introduction begins with "Does the Front Wheel Drive Assist (FWDA) tractor possess enough advantage over the two-wheel drive (2WD) to justify its additional cost? In other words, "do front wheel drivers pay? This question is one that is being asked by farmers contemplating the purchase of a tractor, especially those who are considering 75 kW PTO (100 PTO HP) tractors or larger, for the additional cost may be 15% or more of the purchase price." And, in regard to (2) the Paper contains a printout, dated May 21, 19867, showing the tractive performance of a 104 PTO kW (140 PTO HP) 2WD tractor, equipped to operate "to its best advantage" and pulling in moist to dry sand." The Test Data are telling:
POWER=57 HP (42.23 kW); i.e., amount of drawbar power transmitted to the implement--and the "end product; and produced from two recorded factors, namely PA1 DRAWBAR PULL=5,064 LB (22.52 kN) and PA1 SPEED=4.2 MPH (6.75 km/h) i.e., tractor's ground speed--as opposed to its wheel speed--reduced due to PA1 SLIP=24%; meaning that tractor's wheel speed was no less than 4.2/(1-0.24)=5.5 MPH (8.8 km/h). Also shown in printout is-- PA1 FUEL CONSUMPTION=8.70 gal/h (32.92 L/h) and the resulting Fuel Efficiency=6.51 HPh/gal (1.28 kWh/L). PA1 "When the DB3 was active on the dry soil surface, speed was increased by 62.3%, slip was reduced by 45%, drawbar power was increased by 64%, fuel efficiency was increased by 51% and fuel/km was decreased by 34%. PA1 On the wet, sodded soil, the speed was increased by 107%, slip was decreased by 86%, power was increased by 85%, fuel efficiency was increased by 79%, and fuel/km was reduced 61% by the DB3.".
Data such as the above serve to explain why farmers demand better performance on their 2WD tractor, and the dramatic increase in sales of the FWDA option in spite of its high additional cost. A farmer has no way of knowing that his new 140 PTO HP (PTO HP=Power Take Off Horsepower) 2WD typically delivers a mere 41 percent of its advertised max.HP (57/140), even though equipped with duals of oversize drive tires (20.8-38), and ballasted to weigh no less than 20,516 pounds (9,306 kg) in order to function "to its best advantage". His tractor's drawbar pull is only 25 percent of total tractor weight (5,064/20,516), even though the tractor's four brand new drive tires slip at a 24% rate--meaning that one of four tire revolutions are wasted as tire wear and heat. In consequence, the tractor's Fuel Efficiency (measurable as drawbar power produced for each unit of fuel per hour consumed) is a modest 6.51 HPh/gal (57/8.70) and 1.28 kWh/L (42.23/32.92), approximately half of that obtained on the concrete test course in the official Nebraska Tractor Tests.
While the FWDA device has become a standard option from tractor manufacturers, tire makers are continuing their attempts to improve the characteristics of farm tires, both bias ply and radial ply types. Much of the research is focused on the configuration and spacing of the tire lugs.
In explaining the mediocre performance exhibited in the above-cited ASAE paper, reference is again made to the above-noted U.S. Pat. No. 4,508,150, wherein it is stated that the maximum "terra-dynamic" efficiency averages 65% based on tests conducted by National Institute of Agricultural Engineering, Silsoe, England as reported by Dwyer et al. in 1976. More recent findings by other research institutions indicate that no further significant improvement in traction efficiency has been achieved. This may be a result of the wide variety of tasks that a farm tractor is called upon to perform; it encounters a wide range of soil conditions when pulling implements in fields, and it must have satisfactory tire life and a smooth ride while traveling on concrete roads. It thus can be said that the design of a farm tire becomes an exercise in "compromise" engineering.
A major U.S. tire manufacture, in a recent advertisement, sums up this situation succinctly in referring to its line of farm tires: "Three of the more important tire performance criteria are traction, vibration, and self cleaning ability. Ironically as you improve one aspect, trade offs may occur with the others." As the ad deals with its new high traction tire, it tells about the improvement in traction over other tires. The ad states: "WITH SUCH GREAT TRACTION, WHO WOULD EXPECT REDUCED VIBRATION, TOO"? It explains that vibration was reduced by refinement in lug configuration and lug spacing, along with the already accepted innovation of the so-called "long-bar/short-bar" tread design now offered by most farm tire makers. From this one may conclude that the price for attaining "great traction" is generally a bumpy and uncomfortable ride in the fields.
An article entitled "Long-bar/short-bar: Pulling for a smoother ride" (in the booklet "FARM TIRE HANDBOOK III" by Successful Farming, 1988, Des Moines, Iowa) says, in part, referring to conventional tread bar design: "But, heavy, spaced out lugs on conventional tires, coupled with bigger tractors, quiet cabs and more road travel, often causes serious vibration when tractors are driven on hard surfaces. . . . The resultant impact on the driver can be very disturbing."
Referring to the above-noted ASAE Paper No. 86-1067, the report does not state the traction performance of an accompanying FWDA tractor; it does, however, contain a chart indicating that its fuel efficiency was some 15 percent better than that of the 2WD-tractor. That even large Four-Wheel-Drive (4WD) tractors of the so-called articulated design have performance problems in fields is exemplified by a 1991 brochure published by Caterpillar Inc., with the heading "It's Not How Many Horses Could Pull . . . IT'S HOW MANY DO!." There is cited comparative test data for 4WDs vs Caterpillar's innovative rubber belted agricultural tractor model Challenge 65; . . . conducted by the Center For Agricultural Equipment (formerly the Nebraska Tractor Testing Laboratory) University of Nebraska, Lincoln, during Fall, 1988.
The following table from the Caterpillar brochure describes not only the relative improvement obtained with Caterpillar s so-called Mobil-trac System but also compares its performance on concrete and in soil:
______________________________________ Drawbar Horsepower MPH % Slip ______________________________________ Challenger 65 concrete 206 4.6 1.62 (270 HP) soil 200 4.5 2.55 4 Wheel Drive concrete 213 4.4 6.3 (280 HP) soil 168 3.7 19.9 ______________________________________
The brochure emphasizes that "Four wheel drive tractors convert only 60% of engine power to drawbar power.". This 60% factor is compared with 41% for the regular 2WD in the above. It is noteworthy that the brochure refers to performance in soil; the 168/280=0.60. The brochure further says:
"20% slip on firm soil track."; i.e., the 19.9% shown.
"21% loss of drawbar horsepower from concrete to soil."; i.e., 168/213=0.79.
"16% loss of speed due to traction loss."; i.e., 3.7/4.4=0.84.
And in regard to its Challenger 65, it says, in part, "Nebraska tractor tests conclusively prove the Mobil-trac System on the Challenger 65 delivers the most PULLING POWER." And, "98% of speed retained due to outstanding traction!"; i.e., 4.5/4.6=0.98.
During the 1980s two methods for significantly enhancing traction of agricultural tractors emerged, each employing a different design concept. Clearly, the method of providing a very large "footprint: by means of so-called crawler tractors has been useful since the early days of tractors, albeit with the distinct disadvantage that such vehicles cannot travel on paved roads. This drawback was overcome by replacing the conventional steel track with one of rubber, as introduced in 1987 in the innovative model Challenger 65, discussed in the foregoing. Even though its weight is in order of 31,000 pounds (14,090 kg), its two rubber tracks are large enough to give a ground pressure in the order of 6 pounds per square inch (41.5 kPa), and therefore it is sensitive only to the weaker top layer of the soil. Its 72 rubber grousers per track enable it to transmit 200 drawbar horsepower at only 2.55% slip in soil, less than 1% greater than the slippage on concrete, as is evident from the test data quoted above. This result is contrasted to a nearly fourteen percent difference in the slip rate of the 4WD, also tested. Caterpillar's rubber track concept is thus an effective, albeit costly, method of solving the longstanding traction problem of agricultural tractors. So is the method of utilizing the stronger subsurface layer of soil, having higher "soil values". The above-cited U.S. Pat. No. 4,402,357 includes an analysis, wherein such higher soil values are equated with ballast weight. The effectiveness of the other method which emerged in the 1980's is seen from official tests conducted by University of Arkansas. In ASAE TRANSACTIONS, Vol. 29 (5); Sep.-Oct. 1986, Dr. J. T. Walker reports on the performance of a device, constructed in accordance with the above-cited U.S. Pat. No. 4,508,150 in his article entitled "Dynabite Tractor Tire Attachment Performance in Clay Soil".
The so-called Cone Index is a measure of soil strength and a recent ASAE paper gives an example of how Cone Index increases with soil depth: The article, entitled "Development of a Unique, Mobile, Single Wheel Traction Testing Machine" [ASAE Transaction, Vol. 29(5) Sep.-Oct. 1986; S. K. Upadhyaya et al.] contains a table, entitled "A typical variation of Cone Index with depth", which says, in part, that "average cone index, kPa" is 596 in "Layer 0-50 mm", i.e., the "strength" of the top soil.
By way of example, TABLE 5 in the Walker article shows the difference in performance of a 2WD-tractor. ". . . caused by engagement of the DB3 device relative to performance when disengaged (DB0)", on dry soil and on wet grass. The results are accompanied by the following statement:
The method in accordance with present invention, like the challenger 65, addresses the problems of rubber-tired agricultural tractors, but is far less costly, both in regard to traction per se and in respect to vibration tendency.